In contemporary integrated circuit systems, a precision current reference is typically required to provide a controllable bias source. For many years, analog and digital circuit systems have used a topology known as a "band-gap" current reference. A band-gap current reference uses the energy band-gap property of a semiconductor device to arrive at a predictable and relatively stable voltage reference from which a reference current can be generated. Operationally, a band-gap voltage reference uses the diode voltage characteristic of a bipolar transistor's base-emitter junction to derive a voltage reference. The base-emitter voltage in a bipolar transistor is given by the following expression: ##EQU1## where V.sub.BE is dependent on V.sub.gO, the band-gap voltage; T, the absolute temperature in .degree.Kelvin; T.sub.O, the reference temperature in .degree.Kelvin (usually 300 .degree.K); V.sub.BEO, the reference base-emitter voltage (measured at T.sub.O); n, the emission constant; k, the Boltzmann constant; q, the fundamental unit of electronic charge; I.sub.C, the bipolar transistor collector current; and I.sub.CO, the bipolar transistor collector current (measured at T.sub.O).
In the case where a first and a second bipolar transistor are operated at different current densities J, the difference in their respective base-emitter voltages is given by the expression: ##EQU2## with J.sub.1 and J.sub.2 representing the current densities in each of the respective bipolar transistors. .DELTA.V.sub.BE can be shown to represent a constant differential from V.sub.BEO, and the sum of these two terms is the band-gap voltage as given by the expression: ##EQU3##
This band-gap voltage, particularly the .DELTA.V.sub.BE term, when applied to a pair of base-connected bipolar transistors configured with the first transistor having its emitter connected to ground and the second transistor having its emitter connected to ground via a resistor R (see FIG. 2), results in a current reference with a magnitude of approximately .DELTA.V.sub.BE /R amperes.
The problem with such a conventional band-gap current reference is that the .DELTA.V.sub.BE term is strongly temperature dependent. This along with the temperature dependence of the resistor R, yields a less than desirable situation when extreme accuracy is required over a wide range of temperatures.
Previous attempts to create a current source reference with an adjustable temperature characteristic have resulted in circuits that require a minimum of one resistor to adjust the current reference and at least one other resistor to adjust the temperature coefficient. In these prior art circuits, the alteration of one parameter caused changes in the other, thus resulting in an iterative process for adjustment. In keeping with the present trend of integrated circuit manufacturing, iterative processes for adjustment are not desirable because they increase the cost of the device, decrease reliability, and lower the overall process yield.